This disclosure relates to data storage, and more particularly to an optical data storage system that reads and writes data by coupling optical radiation energy to and from a storage medium.
Optical storage systems can achieve high areal density data storage by using a tightly-focused laser beam to write or read information to and from a suitable storage medium, e.g., an optical disk. An optical storage system typically uses an optical head with a large numerical aperture to focus a monochromatic optical beam to a small spot on a recording layer in the storage medium. The optical head also collects the reflected optical beam from the medium to extract useful control or data signals.
The design and performance of the optical head and the storage medium can be critical to the performance of the storage system, including data recording, data retrieval, and beam tracking.
The systems and methods of this disclosure are in part based the recognition that particles can transfer from the medium surface to the optical head due to heating of the medium surface by the focused optical beam. Such mass transfer can contaminate the optical head and hence can cause signal distortions in many optical disk drives in both the far-field configuration where the optical head and the medium are spaced greater than one wavelength of the radiation and the near-field configuration where the optical head and the medium are spaced less than one wavelength of the radiation. In the far-field configuration, the light is coupled between the head and the storage medium by light propagation. In the near-field configuration, at least a portion of the light coupling is through evanescent fields.
It has been discovered that the mass transfer due to the heating of the medium surface by the focused optical beam exhibits a threshold behavior. The effects of the mass transfer become significant to distort the optical signals and to cause potential damages to the optical head when the optical power density on the medium surface is above a particular threshold value. Hence, the storage medium may be structured to allow sufficient optical power density to reach the storage layer for writing and reading while keeping the optical power density at the medium surface below that threshold power density. This can substantially reduce the adverse effects of the mass transfer. When different mass transfer processes are present in an optical storage system and have different threshold power densities, the medium may be structured to keep the optical power density at the surface below the smallest threshold power density.
An optical storage device according to one embodiment includes a storage medium which has a data storage layer formed on a substrate to interact with radiation energy coupled from an optical head. A transparent capping layer is formed over the data storage layer to have a first surface facing the data storage layer and a second surface opposing the first surface to receive the radiation energy from the optical head. The transparent capping layer is operable to transmit a convergent beam of the radiation energy to focus on or near the data storage layer.
The thickness of the capping layer is set to allow a sufficient spacing between the second surface and the data storage layer so that a beam spot size of the convergent beam on the second surface is sufficiently large to make a power density of the radiation beam on the second surface less than a threshold power density for mass transfer from the medium to the optical head.
These and other embodiments and associated advantages of the present disclosure are set forth in the accompanying drawings and the description below.